1. Field of the Invention
The present invention relates to a fine metal particle-dispersion solution including metals such as noble metal and copper (a metal colloid) and a method for producing the solution; a coating solution for forming electrically conductive films; a conductive film using the solution and a method for forming the film. The fine metal particle-dispersion solution of the present invention is useful for various purposes and particularly useful for transparent films, more particularly for a transparent film for providing a Braun tube and/or a CRT of a TV and/or a computer with an antistatic property for static electricity, a sealing property for electromagnetic waves including ultraviolet rays and infrared rays and an anti-glaring property, which requires a low temperature baking.
2. Description of the Related Art
Since fine metal particles having a mean primary particle size of from several nm to several tens of nms (a metal colloid) can pass rays therethrough, a transparent film can be formed by combining the particles with a binder. Particularly silver fine particles are widely used for the above-mentioned use.
In relation to a transparent film for a Braun tube and a CRT of a TV and/or a computer, it is known that the Braun tube of a TV and a CRT of a computer can be provide with an antistatic property for static electricity and anti-glaring property (protection from the projection of an outer light) by a two layer film formed by an upperlayer of a transparent film having a low refractive index (for example, a film composed of a silica type material) on an underlayer of a transparent film having a high refractive index. The transparent film having the above-mentioned two layers composed of semiconductor fine particles such as ITO (indium oxide doped by tin) and ATO (tin oxide doped by antimony) is disclosed in JP-A 5-290634 and JP-A 6-12920.
Recently, there have developed concerns over bad influences on the human body produced by electromagnetic waves released from Braun tubes and CRTs and error functioning of computers caused by electromagnetic waves from outside thereof, and there have been set in various countries new standards for the emission of electromagnetic waves of low frequency. Consequently an electromagnetic wave-sealing property has been required for Braun tubes and CRTS. For providing the electromagnetic wave-sealing property, it is necessary to form a conductive film having a low resistance of from 102 to 103xcexa9/xe2x96xa1 in terms of surface resistance on the surface of the Braun tube or CRT which is a base. As the underlayers of the transparent film according to the above-mentioned two layers have a low conductivity, it is difficult to obtain the low resistance as mentioned-above.
Accordingly, trials were carried out to satisfy all of the electromagnetic wave-sealing property, antistatic property and anti-glare property by forming the underlayer of transparent film of the two layers using metal particles having a mean primary particle size of up to 0.2 xcexcm (200 nm), in some case, up to 0.05 xcexcm (50 nm) to provide a low resistance. For examples, JP-A 8-77832, JP-A 9-115438, JP-A 9-331183, JP-A 10-74772, JP-A 10-154473 disclose the above-mentioned trials. The fine particles of noble metal are mainly used and the fine particles of Ag are most frequently used as fine metal particles from the standpoint of conductivity.
A particle size having a mean primary particle size of up to 200 nm are within a colloid area. That is, a dispersion solution including metal particles having the mean primary particle size of such a small size is a metal colloid. The metal colloid is hydrophobic. As the fine metal particles as a dispersion have an inferior affinity for water as a dispersion medium, the metal colloid is thermodynamically unstable with the result that aggregation easily arises when an electrolyte exists. Accordingly, it is necessary to add a large amount of a protective colloid (a hydrophilic colloid such as a water-soluble polymer) having a function to stabilize the hydrophobic colloid such that the metal colloid can be stable.
In case of a metal colloid containing a large amount of the protective colloid, the protective colloid which is typically an organic material having no conductivity impedes conductivity when used for forming a conductive film. For that reason, a sufficient conductivity is unavailable without raising the baking temperature for forming the transparent film up to a high temperature which makes it possible completely to dissolve and purge the organic material (for example, higher than 350xc2x0 C.). However such a high baking temperature causes the drop of a phosphor included in the Braun tube, the inferiority of measurement accuracy, the change of vacuum balance due to a gas generation and the corrosion of a electron gun in case of forming a transparent film on a Braun tube or a CRT of a TV and/or computer.
It is known from more than 100 years ago that an aqueous solution of metal salt is reacted with a reduction agent to produce a metal colloid. However, any methods use a large amount of a protective colloid to stabilize the metal colloid except for the method disclosed by Carey Lea in 1889 (M. Carey Lea, American Journal of Science, 37:491, 1989).
According to the Carey method, an aqueous solution of sodium citrate and an aqueous solution of ferrous sulfate are mixed, thereby adjusting the aqueous solution of reduction agent including citrate ion and ferrous ion (that is, aqueous solution of ferrous sulfate), and then the adjusted aqueous solution of reduction agent is mixed with an aqueous solution of silver nitrate to reduce silver nitrate with the result of obtaining a silver colloid. Citrate ions stabilize the colloid adsorbed to fine silver particles such that the silver colloid can be stabilized without adding a polymer protective colloid.
In principle, this method can be used for producing any other noble metal colloid by replacing the aqueous solution of silver nitrate with an aqueous solution of another noble metal salt.
JP-A 10-66861 discloses a silver colloid solution and the method for producing the same, based on the Carey Lea method. According to the method, the aqueous solution of the reduction agent and the aqueous solution of silver nitrate are mixed while stirring at from 1,000 to 10,000 rpm, preferably changing the temperature or stirring speed during the process, thereby to precipitate fine silver particles having various sizes. The precipitated fine silver particles are recovered by centrifugation and the recovered fine particles are dispersed in water such that a solid content of silver is from 1 to 80 wt. % for use as a coating material for forming a transparent conductive film.
According to the Carey Lea method and the method disclosed in JP-A 10-66861, the stabilized colloid can be obtained in case of a silver colloid and some noble metal colloid.
However, the stabilized colloid can not be necessarily obtained by the above-mentioned methods in case of another noble metal and other metals such as Cu. Furthermore, serious problems have been found when a metal colloid is produced using two or more kinds of metals (for example, silver and palladium). That is, according to the above-mentioned methods, different kinds of metals precipitate individually (for example, silver and palladium individually) to form the metal colloid. Accordingly, when the metal colloid is used for a coating material, fine metals particles move during forming a film or baking the film and the fine particles of the same kind of metal is easy to gather each other with the result that there is a tendency to form a film having a nonuniform distribution of different kinds of metals therein. Therefor, a film property differentiates according to part by part of the film with the result that a transparent conductive film having a stable quality can not be obtained.
In addition, when the above-mentioned two layer film is formed using the silver colloid produced by the Carey Lea method and the method disclosed in JP-A 10-66861, the following results have been found: Fine metal particles of the transparent conductive film of the underlayer are subject to changes of particle forms due to surrounding factors such as temperature and humidity. This causes an unstable conductivity of the film and in some cases the film peels off. If the fine metal particles are laid compactly to stabilize the conductivity, the transparency falls steeply, the adhesion property of the film falls remarkably and the film can not be practically used.
As mentioned above, durability such as thermal resistance, humidity resistance, chemical resistance and weather resistance (ultraviolet rays resistance) in the transparent conductive film using the conventional silver colloid is not necessarily sufficient, for example, the film on a Braun tube suffers a secular change and the electric resistance of the film increases gradually with the result that there is a possibility to lose properties required for the transparent conductive film, particularly an electromagnetic wave-sealing function and in some cases the film peels off.
We, inventors have found that the durability of the transparent conductive film formed from the silver colloid can be remarkably improved by mixing palladium, that is, using a metal colloid including fine silver and palladium particles.
However, when the film is formed using the metal colloid including fine silver and palladium particles produced by the above-mentioned conventional methods, a transparent conductive film is formed in which the fine silver particles and the fine palladium particles distribute nonuniformly and a transparent conductive film having a uniform distribution of silver and palladium can not be obtained.
An object of the present invention is to provide a metal colloid (that is, a fine metal particle-distribution solution) from which a transparent conductive film having a uniform metals distribution therein can be obtained when the transparent conductive film including two or more kinds of metals is formed from the metal colloid, and a method for producing the metal colloid.
A further object of the present invention is to provide a coating solution for forming a conductive film which enables the formation of a transparent conductive film having a thermal resistance, humidity resistance, chemical resistance and weather resistance greater than a transparent conductive film formed from the silver colloid such that the above-mentioned problems due to using the silver colloid can be solved, and to provide a low resistance conductive film formed from the coating solution, particularly above-mentioned two layer film having a low reflectivity.
The present inventors have studied a metal colloid including fine particles comprising two kinds of metals and a method for producing the colloid, based on the Carey Lea method for producing a silver colloid and have found that a reaction condition in mixing an aqueous solution of reduction agent and an aqueous solution of metal salt to be reduced has a great influence on the result of the reaction and when the mixing is carried out under an atmosphere having substantially no oxygen such as an inert gas atmosphere, a metal colloid can be obtained which includes fine metal particles produced by precipitation of two metals in the mixed state each other (that is, an alloyed state).
Two metals are alloyed in the fine metal particles of the metal colloid obtained by this method and all the fine particles comprise the same metal composition. Accordingly, when film forming is carried out using this colloid, a transparent forming conductive film can be obtained which reliably has a uniform distribution of the two metals in any part of the film. The evidence that two metals in the fine metal particles of the metal colloid are alloyed is verified by the result that the metal composition ratio of a filtrate is substantially the same as that of a precipitate at any gravitational acceleration when the metal colloid is centrifuged at different gravitational acceleration values (the difference is within the range of 6%).
The above-mentioned method can be applied to a metal colloid comprising two or more (e.g. three, four and five) kinds of metals. In addition, the following findings have been found: A metal colloid can be stably produced by this method in any case of one kind of metal selected from all the noble metal (that is, Au, Pt, Ir, Pd, Ag, Rh, Ru, Os), Re and Cu, and the precipitated metal particles are fine and the particle size scattering of the particles is very small.
Furthermore, the present inventors have found the following findings.: A durability of the film formed from a silver colloid can remarkably be improved by mixing palladium, that is, by using a metal colloid including Agxe2x80x94Pd fine particles. However, when a metal colloid is produced according to the Carey Lea method, Ag and Pd precipitate individually to form a metal colloid and when the metal colloid is used as a coating solution, the fine metal particles move during forming a film or baking the film and the same kind of metal is easy to gather each other with the result that the film having a ununiform distribution of Ag and Pd is produced. In this case, as a good durability can not be obtained and in addition, the properties of the film vary at every part of the film, a transparent film can not be obtained which has a stable quality.
Thereupon, the present inventors have studied a metal colloid including fine metal particles comprising Ag and Pd and a method for producing the colloid, based on the Carey Lea method for producing a silver colloid and have found that a reaction condition in mixing an aqueous solution of reduction agent and an aqueous solution of metal salt to be reduced has a great influence on the result of reaction and when the mixing is carried out under an atmosphere having substantially no oxygen such as an inert gas atmosphere, a metal colloid can be obtained which includes alloyed fine metal particles produced by precipitation of Ag and Pd in the mixed state each other.
Ag and Pd are alloyed in the Agxe2x80x94Pd fine particles of the metal colloid obtained by this method and all the fine particles comprise substantially the same metal composition ratio. Accordingly, when film-forming is carried out using this colloid, a transparent conductive film can be obtained which reliably has a uniform distribution of Ag and Pd in any part of the film. The evidence that Ag and Pd are alloyed is verified by the result that the metal composition ratio of a filtrate is substantially the same as that of a precipitate at any gravitational acceleration when the metal colloid is centrifuged at different gravitational acceleration values (the difference is within the range of 6%).
The first aspect of the present invention includes a method for producing a fine metal particle-dispersion solution and the fine metal particle-dispersion solution produced by this method. The method comprises the following steps
{circle around (1)} adjusting an aqueous solution of metal salt (A) the metal comprising one or more metals selected from the group consisting of Au, Pt, Ir, Pd, Ag, Rh, Ru, Os, Re and Cu;
{circle around (2)} adjusting an aqueous solution (B) including citrate ion and ferrous ion; and
{circle around (3)} mixing solution (A) and solution (B) under an atmosphere comprising substantially no oxygen to produce fine metal particles.
This method optionally includes the steps of recovering fine metal particles from the mixed solution after step {circle around (3)} followed by desalting the fine metal particles and redispersing the desalted fine metal particles in water and/or an organic solvent. In addition, in a preferred embodiment step {circle around (2)} of adjusting the aqueous solution (B) is carried out under an atmosphere having substantially no oxygen and step {circle around (3)} of mixing aqueous solution (A) and aqueous solution (B) is carried out while stirring at from 25 to 95xc2x0 C.
The fine metal particle-dispersion solution of the present invention is a solution in which fine metal particles are dispersed in water and/or an organic solvent, the metal comprising one or more metals selected from the group consisting of Au, Pt, Ir, Pd, Ag, Rh, Ru, Os, Re and Cu, and when the dispersion solution is centrifuged at two or more different gravitational acceleration values, the metal composition ratio of a filtrate is substantially the same as that of a precipitate at any gravitational acceleration (the difference is within a range of 6%).
A second aspect of the present invention includes a coating solution for forming a conductive film which comprises Agxe2x80x94Pd fine particles included in water and/an organic solvent, the Agxe2x80x94Pd fine particles being precipitated by mixing an aqueous solution (A) of a silver salt and a palladium salt and an aqueous solution (B) of citrate ion and ferrous ion under an atmosphere having substantially no oxygen.
The preferable second aspect of the present invention includes the following
{circle around (1)} the Agxe2x80x94Pd fine particles are desalted after precipitation;
{circle around (2)} solution (B) is adjusted under an atmosphere having substantially no oxygen;
{circle around (3)} solution (B) comprises citrate ion and ferrous ion of from one to five moles each relative to a total valence number of metal ion in solution (A), and has a pH 3 to 10;
{circle around (4)} solution (A) and solution (B) are mixed under stirring at from 25 to 95xc2x0 C. such that pH of the mixed solution is from 3 to 9 after mixing and a stoichiometric amount of metal formation is from 2 to 60 g/L.
and/or
{circle around (5)} a Pd/(Pd+Ag) weight ratio in solution (A) is from 0.001 to less than 1 and a primary mean particle size of the fine particles is from 1 to 15 nm.
The coating solution for forming a conductive film of the present invention is a solution in which Agxe2x80x94Pd fine particles are dispersed in water and/or an organic solvent and when the dispersion solution is centrifuged at two or more different gravitational acceleration values, the metal composition ratio of a filtrate is substantially the same as that of a precipitate at any gravitational acceleration (the difference is within the range of 6%).
The coating solution for forming a conductive film of the present invention optionally contains an inorganic binder and/or an organic binder and is preferably provided with pH of from 3.2 to 8.0, an electric conductivity of up to 2.0 mS/cm and a metal content of from 0.1 to 10 wt. %.
According to the present invention, the second aspect further includes a multi-layer conductive film having a low resistance which is provided by forming a Agxe2x80x94Pd fine particle film by coating on a base the coating solution for forming a conductive film of the present invention having no binder, followed by drying the coated solution and then by forming a transparent upperlayer by coating on the Agxe2x80x94Pd film a binder-contained solution, preferably a solution having a silica precursor, followed by drying thereof.
According to the present invention, the second aspect still further includes a multi-layer conductive film having a low reflectivity, a low resistance and an excellent durability which is formed on a base and comprises two layers of an underlayer and upperlayer, wherein the underlayer includes Agxe2x80x94Pd fine particles formed by the above-mentioned coating solution for forming a conductive film of the present invention and the upperlayer comprises a transparent film having a refractive index lower than that of the underlayer, the upperlayer film preferably comprising a silica type material. The base is preferably an image display part of an image display device.